Voltage Controlled Panner

Voltage Controlled Panner

(c) 1994, G. Forrest Cook

Description

This circuit is useful for audio effects and may be used as
a standard plug-in module for voltage controlled synthesizers.
The circuit has one control voltage input, one audio input and
two audio outputs. The control voltage input can be supplied
with a continuously variable voltage from 0V to 10V, 0V pans
the signal to the left output, 5V pans the signal to the center,
and 10V pans the signal to the right output.

Taking the circuit further,
it should be possible to replace the two VCA circuits in this panner with
a National Semiconductor LM13700 dual transconductance amplifier IC
wired as two voltage controlled amplifiers.
See the LM13700 applications notes for details.

Theory Of Operation

The control voltage is fed into the first half of a 1458 op-amp,
this stage inverts the signal and sets the offset and gain for the
right channel gain control circuit.
This signal is then fed into the second half of the 1458 op-amp which inverts
the voltage again and generates the signal for the left gain control circuit.
The 10K trimmer adjusts the Left channel gain, set it for equal gain
from both channels with 5V on the control input.
Each audio stage is identical, only one is shown in the schematic.

The pair of 2N3904 transistors form a differential pair that provides voltage
controlled gain.
The two transistors should be matched, see the note on the schematic.
Each gain control stage is fed into an NE5532 low noise audio op-amp that
is wired as a differential amplifier.

Construction

I recommend building this circuit on perforated circuit board using
point-to-point wiring.
You can also make a PC board if you have the appropriate tools.
Keep the input lead lengths short and provide a filtered +/- 15V power source.
It may be useful put the transistors into sockets, this will allow
different transistors to be matched without a lot of soldering.

Use

Connect the input to an audio signal source, typically an analog
synthesizer. Connect the panner outputs to a stereo amplifier or
mixing board inputs.
Apply various wave-shapes to the control voltage input, listen to the
sound moving around.

Sine waves provide a nice control signal and I have
experimented with a microprocessor controlled digital to analog
converter (DAC) that can generate arbitrary wave-shapes with
interesting results.
See my
Panner Waveform Generator for details.
If you have a modular synthesizer (lucky
you), an interesting thing to do is send a triggered modulation
wave to the panner, try using the wave that controls the VCF.
A triggered, amplitude-modulated sine wave makes for a great effect.

If you want to hear the panner circuit in action,
it is used in the CD "Soundtrack for a Low-Budget Sci-Fi Movie"
available on my Fractal Music page.